No cure currently exists for rheumatic diseases. Rather, therapeutic agents are used to treat the symptoms. Typically, the therapeutic agents are administered over long periods of time and the therapeutic value is often diminished by adverse side effects.
Rheumatic diseases encompass a group of diseases that affect the musculo-skeletal and connective tissues of the body. These diseases are characterized by chronic inflammation that often leads to permanent tissue damage, deformity, atrophy and disability. Rheumatic diseases affect the joints, bone, soft tissue, or spinal cord (Mathies, H. 1983 Rheuma) and are classified as inflammatory rheumatism, degenerative rheumatism, extra-articular rheumatism, or collagen diseases. Some rheumatic diseases are known to be autoimmune diseases caused by a subject's altered immune response.
Rheumatoid arthritis is a progressive rheumatic disease, affecting approximately 2% of the adult population of developed countries (Utsinger, P. D., et al., 1985 Rheumatoid Arthritis, p. 140). This disease is characterized by persistent inflammatory synovitis that causes destruction of cartilage and bone erosion, leading to structural deformities in the peripheral joints. The symptoms associated with rheumatoid arthritis include joint swelling, joint tenderness, inflammation, morning stiffness, and pain, especially upon flexing. Subjects having advanced stages of arthritis suffer from structural damage, including joint destruction with bone erosion (in: “Principals of Internal Medicine, Harrison, 13th edition, pages 1648-1655). In addition, patients can present other clinical symptoms of various organic lesions, including lesions of the skin, kidney, heart, lung, central nervous system, and eyes due to vasculitis related to the autoimmune process.
Other symptoms that correlate with rheumatoid arthritis include elevated erythrocyte sedimentation rates, and elevated levels of serum C-reactive protein (CRP) and/or soluble IL-2 receptor (IL-2r). The erythrocyte sedimentation rate is increased in nearly all patients with active rheumatoid arthritis. The level of serum C-reactive protein is also elevated and correlates with disease activity and the likelihood of progressive joint damage. Additionally, the level of soluble IL-2r, a product of activated T-cells, is elevated in blood serum and synovial fluid of patients with active rheumatoid arthritis (see: “Principals of Internal Medicine, Harrison, 13th edition, page 1650).
Rheumatoid arthritis is believed to be a T-cell-mediated autoimmune disease, involving antigen-nonspecific intercellular interactions between T-lymphocytes and antigen-presenting cells. In general, the magnitude of the T-cell response is determined by the co-stimulatory response elicited by the interaction between T-cell surface molecules and their ligands (Mueller, et al., 1989 Ann. Rev. Immunol. 7:445-480). Key co-stimulatory signals are provided by the interaction between T-cell surface receptors, CD28 and CTLA4, and their ligands, such as B7-related molecules CD80 (i.e., B7-1) and CD86 (i.e., B7-2), on antigen presenting cells (Linsley, P. and Ledbetter, J. 1993 Ann. Rev. Immunol. 11:191-212).
T-cell activation in the absence of co-stimulation results in anergic T-cell response (Schwartz, R. H., 1992 Cell 71:1065-1068) wherein the immune system becomes nonresponsive to stimulation.
Since rheumatoid arthritis is thought to be a T-cell-mediated immune system disease, one strategy to develop new agents to treat rheumatoid arthritis is to identify molecules that block co-stimulatory signals between T-lymphocytes and antigen presenting cells, by blocking the interaction between endogenous CD28 or CTLA4 and B7. Potential molecules include soluble CTLA4 molecules that are modified to bind to B7 with higher avidity than wildtype CTLA4 (the sequence of which is shown in FIG. 23) or CD28, thereby blocking the co-stimulatory signals.
Soluble forms of CD28 and CTLA4 have been constructed by fusing variable (V)-like extracellular domains of CD28 and CTLA4 to immunoglobulin (Ig) constant domains resulting in CD28Ig and CTLA4Ig. A nucleotide and amino acid sequence of CTLA4Ig is shown in FIG. 24 with the protein beginning with methionine at position +1 or alanine at position −1 and ending with lysine at position +357. CTLA4Ig binds both CD80-positive and CD86-postive cells more strongly than CD28Ig (Linsley, P., et al., 1994 Immunity 1:793-80). Many T-cell-dependent immune responses have been found to be blocked by CTLA4Ig both in vitro and in vivo. (Linsley, P., et al., 1991b, supra; Linsley, P., et al., 1992a Science 257:792-795; Linsley, P., et al., 1992b J. Exp. Med. 176:1595-1604; Lenschow, D. J., et al. 1992 Science 257:789-792; Tan, P., et al., 1992 J. Exp. Med. 177:165-173; Turka, L. A., 1992 Proc. Natl. Acad. Sci. USA 89:11102-11105).
To alter binding affinity to natural ligands, such as B7, soluble CTLA4Ig fusion molecules were modified by mutation of amino acids in the CTLA4 portion of the molecules. Regions of CTLA4 that, when mutated, alter the binding affinity or avidity for B7 ligands include the complementarity determining region 1 (CDR-1 as described in U.S. Pat. Nos. 6,090,914, 5,773,253, 5,844,095; in copending U.S. patent application Ser. No. 60/214,065; and by Peach et ,1, 1994. J. Exp. Med., 180:2049-2058) and complementarity determining region 3 (CDR-3)-like regions (CDR-3 is the conserved region of the CTLA4 extracellular domain as described in U.S. Pat. Nos. 6,090,914, 5,773,253 and 5,844,095; in copending U.S. patent application Ser. No. 60/214,065; and by Peach, R. J., et al J Exp Med 1994 180:2049-2058. The CDR-3-like region encompasses the CDR-3 region and extends, by several amino acids, upstream and/or downstream of the CDR-3 motif). The CDR-3-like region includes a hexapeptide motif MYPPPY (SEQ ID NO:20) that is highly conserved in all CD28 and CTLA4 family members. Alanine scanning mutagenesis through the hexapeptide motif in CTLA4, and at selected residues in CD28Ig, reduced or abolished binding to CD80 (Peach, R. J., et al J Exp Med 1994 180:2049-2058).
Further modifications were made to soluble CTLA4Ig molecule. by interchanging homologous regions of CTLA4 and CD2B. These chimeric CTLA4/CD28homologue mutant molecules identified the MYPPPY hexapeptide motif (SEQ ID NO:20) common to CTLA4 and CD28, as well as certain non-conserved amino acid residues in the CDR-1- and CDR-3-like regions of CTLA4, as regions responsible for increasing the binding avidity of CTLA4 with CD80 (Peach, R. J., et al., 1994 J Exp Med 180:2049-2058).
Soluble CTLA4 molecules, such as CTLA4Ig, CTLA4 mutant molecules or chimeric CTLA4/CD28 homologue mutants as described supra, introduce a new group of therapeutic drugs to treat rheumatic diseases.
Present treatments for rheumatic diseases, such as rheumatoid arthritis, include administering nonspecific cytotoxic immunosuppressive drugs, such as methotrexate, cyclophosphamide, azathioprine, cyclosporin A, and tumor necrosis factor-alpha (TNFα) blockers or antagonists. These immunosuppressive drugs suppress the entire immune system of the subject, and long-term use increases the risk of infection. Moreover, these drugs merely slow down the progress of the rheumatoid arthritis, which resumes at an accelerated pace after the therapy is discontinued. Additionally, prolonged therapy with these nonspecific drugs produces toxic side effects, including a tendency towards development of certain malignancies, kidney failure, bone marrow suppression, pulmonary fibrosis, malignancy, diabetes, and liver function disorders. These drugs also gradually cease being effective after about 2-5 years (Kelley's Textbook of Rheumatology, 6th Edition, pages 1001-1022).
Alternatively, therapeutic agents that are non-specific immunosuppressive and anti-inflammatory drugs have been used to obtain symptomatic relief. These drugs are dose-dependent and do not protect from disease progression. These drugs include steroid compounds, such as prednisone and methylprednisolone. Steroids also have significant toxic side effects associated with their long-term use. (Kelley's Textbook of Rheumatology, 6th Edition, pages 829-833).
Thus, current treatments for rheumatoid arthritis are of limited efficacy, involve significant toxic side effects, and cannot be used continuously for prolonged periods of time.
Accordingly, there exists a need for treatments that are effective and more potent for treating rheumatic diseases, such as rheumatoid arthritis, and avoids the disadvantages of conventional methods and agents, by targeting a pathophysiological mechanism of auto-immunity.